Effect Of Interfacial Features Of Friction Blocks On The Squeal Noise And Wear Behaviors Of Train Railway Brake

Brake squeal noise generated by the unstable vibration of the brake friction interface causes serious environmental pollution,and seriously affects the comfort and experience of passengers during the train brake process,becoming an important factor restricting the continued promotion and development of high-speed trains.With the continuous improvement of train operation speed,the contact interface between brake disc and brake pad is located at an open environment with stronger friction action and higher thermal stress,which puts forward more stringent standards for the manufacturing requirements for acquiring the low noise and great tribological performance.At present,most researchers are attempt to adjust the ingredients of friction material or its proportion for improving the braking performance of high-speed trains.They mainly focus on the effect of material self-properties on the brake noise and wear behavior of friction interface,while ignoring the effect of the complex structure of brake system and the various interfacial features of friction blocks.Generally speaking,the brake pad of high-speed train is composed of multiple friction block,and their interfacial features such as shape,structure and distributed configurations not only affect the contact characteristics of frictional interface,and also the interfacial force,wear debris generation,friction heat distribution and so on,these changed factors will play an important role in the brake noise characteristics and wear behavior.Therefore,from the perspective of interfacial tribology,the effect of shape,features of leading edges,and the perforated structure of friction blocks on the noise characteristics and wear behaviors of high-speed railway brake system and its influence mechanism have been investigated systematically,which is based on a small scale high-speed railway brake dynamometer.The main research work and conclusions are as follows:(1)A series of brake squeal experiments are conducted by using the friction pair samples which are manufactured from a real high-speed train powder metallurgy brake pad and a forged brake disc.And combining some signal processing methods such as sound pressure analysis,vibration amplitude analysis,and spectral analysis and so on,the effect of braking parameters on the squeal characteristics of brake system and its influence mechanism are investigated systematically and deeply.The results show that the braking parameters have an important influence on the generation and evolution characteristics of vibration and noise of the brake system,and the corresponding noise frequency components display a great difference under the different parameter conditions.The probability of noise occurrence and the levels of sound pressure have the increased trend when the disc rotation speed increases within a certain range.However,the brake system presents a most unstable status in spite of the disc rotation speed under a certain braking pressure.In addition,the variations of braking pressure will cause the changes of contact position between the brake disc and brake pad,which may be one of the important reason why the frequency characteristics of squeal noise presents a huge difference under the different braking pressures.(2)The effect of interfacial shape features of friction blocks on the squeal noise characteristics and wear behaviors of train brake interface is investigated under the dry and wet frictional environments,and the influence mechanism of friction block interfacial shape on the brake interface squeal noise and the correlations of interfacial vibration with the wear features are explored in this part.The powder metallurgy material is used for the manufacture of friction block samples which possess different sectional shapes and the same cross sectional areas,and brake experiments are conducted under the two interfacial frictional environments.The results show that all three block samples exhibits successive vibration during the tests under dry conditions,and this phenomenon may have resulted from the concentrated contact stress at the leading edge of friction block.The triangular block displays the strongest vibration and noise among the three blocks,because the frictional leading edge of triangular block is located around an angle,and corresponding contact stress presents more concentrated.Under the wet condition,the successive vibration has been disturbed due to the presence of moisture.no oscillations were observed in the vibration signals of the circular block,and intermittent oscillations were observed for the triangular and hexagonal blocks.In addition,it is found that the vibration enhancement of the braking interface will aggravate the interface wear by observing the vibration intensity and wear morphology of the three block samples.(3)Taking the triangular block as the research object,the influence mechanism of the friction block leading edge features on the vibration and noise of braking interface and the relationship between the interfacial thermal distribution and wear behavior are systematically studied in this part.Drag-brake tests are conducted by using the friction blocks with different mounting directions,i.e.the different leading edge features.The results show that the brake system presents a low-frequency noise if only one edge of the triangular block is set up as the leading edge,while the system presents a larger vibration intensity and a higher sound pressure energy when an angle of the triangular block is mounted to the frictional leading edge and enters the friction area firstly.It has been improved that the concentrated contact pressure of block surface and higher intensity vibration and sound pressure after enlarging their chamfer of the triangular block.This furtherly proved that the concentrated stress located at the leading edge of friction block is the important reason that caused the squeal noise of braking interface.In addition,the wear analysis shows that the third body frictional layer becomes the main contact platform in the area where accumulates wear debris.Simultaneously,accumulated wear debris increase the real contact area between the friction pairs,which increases the friction force at this area,and consequently cause the quickly rise of the surface temperature of this area.(4)The perforated block and the imperforate block are used to the brake experiments to contrast and investigate the effect of perforated structure of friction block on the interfacial wear debris,and then the wear behavior,interfacial thermal distribution and noise characteristics.The results show that the existence of the circular hole in the middle of the friction block can entrap wear debris and affect the distribution of the wear debris on the block surface,which can significantly affect the wear situations of block surface.The surface regions with serious accumulation of wear debris can lead to the high surface temperature.This can be attributed to the repeated grinding of wear debris and the increase of real contact area,which will further increase the friction force of this regions.While the surface regions with a small quantity of wear debris accumulation and more primary plateaus are found to experience relatively mild wear,and more graphite flakes which plays a role of lubricant presents in this region,thus the corresponding friction force has decreased,and consequently lead to the low surface temperature.By contrasting the experimental results of the two kinds of friction blocks,it is concluded that the perforated block undergoing a slight wear at the area around the leading edge present relative low sound pressure levels.(5)Similar to the fourth research work,the perforated and imperforate block are used to this work to study the effect of perforated structure of friction block on the distribution of interfacial sand,and then the other tribological behavior of braking interface.The evolution trends of the surface wear behavior,thermal distribution situations and squeal noise characteristics of the two kinds of friction blocks under the sandy condition is discussed.The results show that the imperforate block surface experience the more serious wear situation than that under the dry condition,and more contact plateaus are damaged,presenting the squeal noise with a higher intensity.At the same time,the high-frequency vibration with high intensity makes less interfacial wear debris piled up in the block surface,thus the block surface shows a relative low average temperature.However,due to the existence of the middle hole,the perforated block collects a large amount of wear debris,which will gradually accumulate in the friction ring which covered the hole during the braking process,forming the sandy frictional layer,which becomes the main bearing platform at the braking interface.This frictional layer can result in the high temperature on the material surface.At the same time,the interfacial friction action from a large amount of sand brings serious abrasive wear to the block surface,resulting in a large range of material exfoliations.